Femoroacetabular impingement is a condition that has only recently been recognised as a cause of hip symptoms. It is thought to be the underlying cause for >75% of all cases of osteoarthritis of the hip. 
Impingement results either because the femoral head is not quite spherical or because the acetabulum is too deep; thus when the hip is flexed fully the neck of the femur is pushed against the labrum and acetabulum at the front of the hip, causing damage to these structures.
There are 2 basic types of Hip Impingement:
- CAM impingement occurs when the femoral head is not spherical.
- PINCER impingement occurs due to an excess of growth of the upper lip of the acetabular cup.
- If both types of impingement are present it is termed MIXED impingement.
The result of any of these deformities is:
- Increased friction
- Loss or reduction of hip function
Anterior femoroacetabular impingement usually presents in young athletic patients less than 50 years old and involved in activities that require repetitive hip flexion. These patients frequently complain of hip discomfort with sitting and hip flexion activities. The location of the discomfort is predominantly in the groin (anterior inguinal), but can be associated with buttock and lower lumbar discomfort. Anterior femoroacetabular impingement is consistently associated with anterior labral pathology. Therefore, patients may complain of mechanical symptoms (locking, catching, and giving way) indicative of labral tears or articular cartilage delamination lesions. Patients with more severe deformity may also complain of restricted hip motion, specifically limited hip flexion and limited internal rotation in flexion.
On physical examination, the patient’s gait is either normal or slight limp will be present occasionally. A Trendelenburg’s test may be positive, especially if the disease is more established. Abductor strength is routinely assessed and commonly reveals slight weakness. Hip motion should be evaluated very carefully. A restriction of hip flexion and hip internal rotation is quite common. Many of these patients have hip flexion limited to 90-100 degrees (normally 120-130 degrees). Internal rotation in 90 degrees of flexion is quite restricted and is usually between 0 and 10 degrees. This restricted internal rotation in hip flexion is due to osseous impingement of the anterolateral femoral head-neck junction with the acetabulum. The anterior impingement test is almost universally positive and should reproduce the symptom of groin pain.
Posterior femoroacetabular impingement is more common as the disease progresses and a posteroinferior traction osteophyte develops which can produce clinical symptoms of posterior impingement in extension. Posterior impingement of the hip is assessed with the patient in a prone position. The hip is extended and externally rotated to produce posterior impingement of the head-neck junction with the posteroinferior rim of the acetabulum. 
Therapeutic management comprises acetabulo- and femoral osteochondroplasty, which may be administered by open or minimally invasive open surgery as well as arthroscopically. In certain cases impingement pathology has to be addressed by acetabular reorientation (e.g. osteotomy). Published data show that timely diagnosis and treatment is pivotal for therapeutic success.
Before (A) and after (B) osteochondroblasty for a 26 year old female with cam impingement. 
 Clohisy J.C., McClure J.T. (2005). Treatment of Anterior Femoroacetabular Impingement with Combined Hip Arthroscopy and Limited Anterior Decompression. Retrieved on 13 September 2015 from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1888771/
 Fraitzl CR., Kappe T., Reichel H. (2010). Das femoroacetabuläre Impingement – eine häufige Ursache des Leistenschmerzes beim Sportler. Deutsche Zeitschrift fuer Sportmedizin, Jahrgang 61, Nr 12, 2010
 Hip & Knee Advice. (2003). Hip impingement. Retrieved from http://www.hipandkneeadvice.com/index.php/hip-conditions/hip-impingement/